A number of semiconductor technologies involve aligning two or more wafers and thereafter bonding the wafers. Consider, e.g., the use of eutectic bonding, which applies heat and pressure to connect two or more aligned semiconductor wafers. A sufficient alignment is one in which the features from one wafer are properly aligned to corresponding features on a second wafer. Here, alignment tolerances (the dividing line between sufficient and insufficient alignments) are roughly proportional to the sizes of the features on the wafers. One method of alignment involves the use of two or more “flags,” which are markers placed between wafers. After a sufficient alignment is achieved, the flags are removed by a flag-out mechanism, such as a gas-operated mechanism (e.g., using air, N2, etc.) or an electromagnetic mechanism. Removing the flags, however, can result in an insufficient alignment of the wafers, which could cause the entire process to result in unsatisfactory bonding of the wafers.
To minimize unsatisfactory bonding, operators take certain precautions to monitor removal of the flags to ensure that sufficient alignment is maintained. In one approach known to the inventors, an operator visually inspects the alignment to ensure a sufficient alignment is maintained during removing the flags. The operator watches each of the flag-out movements in the mechanism during the removal process and perceptible differences in flag velocity would indicate a misalignment of the wafers. After noticing an insufficient alignment, the operator could respond, e.g., by adjusting the apparatus (e.g., adjusting the gas pressure in a gas-operated mechanism) in order to modify the flag-out procedure based on the perceived differences in flag velocity. However, this method merely makes it possible to achieve a perceptible level of alignment precision. If the differences in each flag's velocity are imperceptible to the operator, not only could a latent, imprecise alignment result, but thereafter bonding the imprecisely aligned wafers would yield an unsatisfactorily bonded set of wafers, which would tend to be scrapped. In other words, an operator's limits of perception can adversely affect a scrap rate (i.e., percentage of unsatisfactorily bonded wafers) of a production and the size of the features on the wafers of a production.